The invention relates generally to radio frequency antennas, and, more particularly, to a phase scanned microstrip series array antenna.
The concept of using microstrip patches of various widths to form a series antenna array is well known to the art. Typically, the series antenna arrays are formed by photo-etching processes from a laminated starting material consisting of a dielectric substrate layer laminated between two conductive sheets. Typically, one of the conductive sheets is used as the ground or reference plane of the microstrip antenna, while the other opposite conductive sheet is photo-etched to form a plurality of microstrip patches connected in series by transmission line segments, as for example described in U.S. Pat. No. 4,180,817, issued Dec. 25, 1979 to Sanford.
When rectangular microstrip patches are utilized in the series antenna arrays, the conductance G of each patch is a function of the printed strip width. The ratio G/G.sub.o of this conductance G with G.sub.o, the line conductance of the transmission line segment supplying r.f. energy to the patch, determines the quantity of power radiated from the edges of the patch. To establish a desired distribution of radiated power, each patch element in the array must be configured to have a specific conductance as a function of its position in the array.
In general, beam scanning of a microstrip series antenna array is accomplished by introducing signal phase shifts between the patch elements of the array so that the main beam can be aimed at a given direction. In the past, this has been done by either changing the operating frequency, which changes the electrical path length between the patch elements, or by inserting phase shifting devices in the signal path between the patch elements.
Until the present invention, it has been difficult to design a beam scanned microstrip series antenna array having a narrow main beam and low side lobes, because of the following design considerations:
(1) To avoid excessive phasing and moding, the width of each patch element must be narrower than a wavelength in the dielectric material, preferably narrower than one half wavelength. However, in order for the series antenna array to be 100 percent efficient, the last patch element in the series of patch elements must have an impedance equal to the impedance of the transmission line connecting these elements, typically 50 to 100 ohms. To achieve this impedance, the last patch element generally must have a width which is much greater than one half the wavelength in the dielectric material, which, in turn, results in excessive phasing and moding. PA1 (2) Since each patch element has a different insertion phase, a uniform patch separation cannot be used. Also, the resonant length of each patch element is different because of the presence of the patch phase. Thus, beam scanning by varying the frequency would be minimal before all semblance of the array properties are destroyed by the presence of the various patch phases. On the other hand, since each patch has a different insertion phase, scanning by means of applying external phasing to each patch element requires different circuitry for each patch element.